The main objective of our study is to test the conditioned neurotransmitter hypothesis which states that drugs can act as unconditioned stimuli for conditioned alterations in neurotransmitter functoning. Previously it has been shown that psychotropic drugs can act as unconditioned stimuli for the establishment of neurotransmitter conditioned reflexes in the rat (Perez-Cruet, 1974, 1976). An additional objective of our research is to investigate the generality and specificity of such a hypothesis. A Pavlovian paradigm will be employed with a sound as a conditioned stimulus and a drug as an unconditioned stimulus. Agonist or antagonist drugs known to have selective neuropharmacological effects on monoaminergic systems will be employed as drug unconditioned stimuli. The criteria of neurotransmitter conditioning will be based on the appearance of changes in neurotransmitter metabolism to a conditioned stimulus that has been previously reinforced with the drug US, but that during the test trial is reinforced with saline.